Limb development may be divided into morphogenetic and active cytodifferentiation phases. During the morphogenetic phase the mesoderm cells of the limb bud have a definite set of morphogenetic properties which are responsible for the elaboration of limb form. These properties are transient in nature. Their loss coincides with the onset of active cytodifferentiation of the cells. Nothing is known about the molecular basis of these morphogenetic properties. The overall objective of this investigation is to obtain information on the underlying molecular events which are responsible for the morphogenetic limb properties. The developing chick limb bud is being used as a model system. Objectives of the investigation are the following: 1. To determine if protein and RNA synthesis play roles in the loss of the morphogenetic properties. Inhibitors of protein RNA synthesis are being used. 2. To determine the role of mitosis in the loss of the morphogenetic properties. 3. To determine if cytodifferentiation plays a role in the loss of the morphogenetic properties. BUdR, an inhibitor of cytodifferentiation, will be used. 4. To map specific areas of the limb bud mesoderm for the time of loss of the morphogenetic properties.